1. Field of the Invention
This invention relates to a novel method of producing interpenetrating polymer network (“IPN”) sheeting. Particularly, this invention relates to an improved method of producing such sheeting for use as various types of wound dressings and scar management products.
2. Description of the Prior Art
U.S. Pat. No. 4,832,009 issued May 23, 1989 to Mark E. Dillon and assigned to Bio Med Sciences, Inc., and U.S. Pat. No. 5,980,923, issued Nov. 9, 1999 to Mark E. Dillon, also assigned to Bio Med Sciences, Inc., Allentown, Pa., the disclosures of which are incorporated herein by reference, describe wound dressings and scar management materials comprised of IPN sheets and in particular IPN sheets of polydimethylsiloxane (“PDMS”) and polytetrafluorethylene (“PTFE”). Interpenetrating polymer networks are defined as a blend of two or more polymers where each material forms a continuous network, each network interpenetrating the other (Sperling, Interpenetrating Polymer Networks and Related Materials Plexem Press, New York, 1981). An IPN is therefore a type of polymer/polymer composite. In each of the above referenced patents a process is used for creating PTFE/PDMS IPN sheeting in which a liquid PDMS composition is impregnated into a microporous or expanded PTFE (“ePTFE”) membrane. This process involves casting the liquid PDMS onto the ePTFE membrane or alternatively casting the liquid PDMS onto a carrier substrate and laying-down the ePFTE membrane onto the Liquid PDMS layer. In either case natural wicking occurs by way of capillary action to effect impregnation of the PDMS liquid layer into the ePTFE membrane. The resultant IPN sheet is then exposed to heat or some other method of vulcanization causing the liquid PDMS to crosslink into a solid elastomer or gel thereby creating the IPN sheet.
This technique may be employed using a static system for single sheets of IPN or a continuous system to produce rolls of IPN of almost any desired length. For thermally crosslink PDMS formulations an oven is typically used to effect crosslinking of the PDMS. In the static system approach a closed convection oven may be employed. For the continuous method the IPN sheets may be passed through a tunnel style oven and wound into a roll.
The above methods are used by the Applicant to produce a variety of IPN sheets. For example, for the production of wound dressing sheets, a thin (0.002 in. or 50 microns) PTFE/PDMS IPN sheet is produced in continuous rolls and then coated with an additional layer of PDMS to provide an adhesive surface to one side of the IPN sheet. This is accomplished by passing the IPN sheet through the system twice; first to create the IPN sheet and second to impart increased adhesiveness by adding a layer of additional PDMS to the skin-contacting side of the IPN sheet.
Another example is for the production of scar management sheets. In this case, a relatively thick layer 10 (0.025 in. or 635 microns) of liquid PDMS is cast onto a carrier substrate 20 (FIG. 4) and the ePTFE membrane 30 is laid on top of the liquid PDMS layer 10 and the impregnation process is effected by capillary wicking. This technique provides a sheet 12 comprising a layer of IPN and a layer of pure PDMS, this creating one side of the sheet that is more adhesive than the other while using a single-pass process. This is due to the extent to which the wicking process occurs. The small pores within the ePTFE membrane 30 act as capillaries and essentially pull the liquid PDMS into the void spaces of the membrane 30 due to the surface tension force between the capillary walls and the liquid PDMS. For thin membranes 30 with small pore sizes this force is enough to bring the liquid to the distal or upper surface 25 of the membrane 30 at which point the surface tension force within the capillaries is relieved and the wicking process stops. The resultant sheeting material is essentially 100 percent PDMS on the skin-contacting surface 15 while the distal or upper surface 25 is comprised of the IPN polymer blend structure.
While the difference between adhesiveness levels of the two surfaces 15 and 25 can be significant, there are cases where even greater disparity is desired. For example, the Applicant produces a scar management product (marketed as Oleeva® Fabric; OLEEVA®is the trademark of Bio Med Sciences, Inc., Registration No. 2,446,261, registered Apr. 24, 2001, for topical sheeting for the prevention and management of dermal scars) where the distal or upper surface 25 of the IPN sheet 12 is bonded to a textile fabric 40 so that garments worn over the product slide easily over the textile fabric 40 and any tendency for the sheet 12 to roll-up or cling to such garments is reduced or eliminated. This design is accomplished by using a two-pass process whereby the IPN sheet of the previous example is coated on the IPN upper surface 25 with an additional layer 50 of liquid PDMS onto which the textile fabric 40 is laid. The additional layer 50 (FIG. 1) of liquid PDMS serves as glue and bonds the fabric 40 to the IPN upper surface 25. While a two-pass process is undesirable for obvious reasons (including economic factors such as processing time and handling expenses as well as quality aspects such as increased defects), numerous attempts at reducing the Oleeva® Fabric process to a single pass have been unsuccessful.
For continuous methods one obvious approach to reducing the process to a single pass would to be to install a second coating station between two sections of the tunnel oven and apply the second layer 50 of PDMS and the textile fabric 40 in-line. This approach was not an option to the Applicant due to the additional floor space required to install a second tunnel oven. Simply cutting the existing oven in half and installing a second coating station in the middle would not have been a desirable approach because this would effectively cut the line speed of the process in half. The crosslinking reaction of the PDMS is dependent on dwell time in the oven. A shorter oven means a slower line speed to maintain the same dwell time. Since the first layer 12 of IPN sheet must be sufficiently cured before it contacts any machine parts or rollers (or it will have a tendency to stick and/or cause the PDMS to migrate) the line speed would have to be reduced by half to provide a material capable of being passed through the rollers required for the second coating process. This approach would therefore essentially double the time required to process the material and require a significant capital investment in a second coating station.